Corvo, Wanted Assassin

Assassinate:{3}{b}{b}Destroy target creature,
Only if you control a White Rat Food-Creature
token.
Plague swarm:{x}{w}{w}{t}Summon X 1/1 White
Rat Food-Creature token with "{t}sacrifice this
creature gain 3 life"
{2}{w/b}{w/b}{ut} Corvo, Wanted Assassin gets
+1/+1 for each Rat you control until your next end step.